Clinical Significance of Mucin Balls

Figure 1: Contact lens wearer displaying large numbers of mucin balls

In assessing the significance of these features, the first task for the clinician is to make sure that they are indeed looking at mucin balls, rather than the features with which they are most commonly confused, namely microcysts or vacuoles. Among the signs to look for when making the differential diagnosis are that mucin balls have a pre-epithelial location, and will usually disappear within a few blinks after lens removal. Both microcysts and vacuoles are intra-epithelial, and persist when the lens is removed. If doubt still remains, instillation of sodium fluorescein will show pooling of the dye (though not frank staining), in the epithelial depressions left behind by the mucin ball (Table 1). This appearance is similar to “dimple veiling”, which was caused by bubbles of gas collecting beneath rigid lenses.

Our knowledge of the nature of mucin balls has improved recently as samples have been collected and subjected to scanning electron microscopy, histochemistry and other analytical techniques.³ These studies have shown it to be quite unlikely that mucin balls have been misnamed, which is to say that their major constituent does indeed appear to be mucin! Presumably, this material is derived from the pre-ocular tear film and is formed into spheroids by the relative motion between the contact lens and corneal surface. Consequently, it has been proposed that reducing this motion, by more closely matching lens shape to that of the ocular surface, would be a possible means of cutting down mucin ball numbers.¹

Despite being squeezed between the contact lens and epithelium, mucin balls tend to maintain their spherical shape. This resistance to deformation is responsible for creating the epithelial depressions and these may be quite deep. When viewed with the confocal microscope, the indentations can often be seen extending at least to the level of Bowman’s membrane⁴, and possibly even beyond.⁵

In these circumstances one might reasonably wonder what happens to the corneal cells immediately beneath a mucin ball? So far, it has proved difficult to visualize epithelial cells in this region; suggesting either, that they have become highly compacted, or possibly that migration away from the local pressure point has occurred. Interestingly, a similar conundrum currently surrounds the mechanisms of epithelial remodeling during orthokeratology, and in that case, recent data tend to favour the compaction theory.⁶

With respect to stromal tissue, indications from rabbit eyes are that the keratocytes or fibroblasts underneath deeper mucin ball impressions show signs of increasing their proliferation rate.⁴ The significance of this finding currently remains unclear, though stromal cells usually divide rather slowly.

While evidence of cellular activity in the cornea is certainly a good reason for continued research, from the viewpoint of both clinician and wearer, it is comforting that there have been almost no reports of associated negative clinical events. This is despite the fact that mucin balls are highly prevalent among silicone hydrogel wearers. Apart from a statistical indication that extended wearers with larger numbers may have a slightly increased risk (x 1.7) of developing a contact lens induced peripheral ulcer (CLPU),⁷ mucin ball related adverse events have not featured in the literature. As there are generally no adverse effects on either vision or subjective comfort,^{1, 8}, clinical intervention is usually unnecessary in cases where mucin balls are the only observable complication. Naturally however, continued, regular review is recommended throughout the period of contact lens wear.

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2. Sweeney DF, du Toit R, Keay L, Jalbert I, Sankaridurk PR, Stern J, Skotnitsky C, Stephensen A, Covey M, Holden BA & Rao G. Clinical performance of silicone hydrogel lenses. In: Silicone Hydrogels, continuous wear contact lenses. Ed: Sweeney DF. 2^nd Edition, Butterworth Heinemann. p 198.
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7. Naduvilath TJ. Statistical modeling of risk factors associated with soft contact lens related corneal infiltrative events. PhD Thesis (Newcastle: University of Newcastle), 2003.
8. Morgan PB & Efron N. Comparitive clinical performance of two silicone hydrogel contact lenses for continuous wear. Clin Exp Optom., 2002; 85, 183-192.